Approaching the problem mechanically is perfect, and saying 'well if X doesn't work what about Y or Z' is the right approach. If you lead with that he may have some refinements or suggestions. When he hears that you're open to this, being helpful, I'm sure he'll have ideas as well and you can figure something out.

Depending on what he's experiencing, he may be able do a one word answer, and then he can give you a word that he can rehearse that's code for 'Yo I'm calling out for today, but can't actually get those words out'. If he's able to give a yes / no, then he also could call you with silence, but then you see that he's the caller and say "Hey [name] I don't hear anything, are you in a shutdown and trying to call off?" and he can respond Yes (listening comprehension is often less affected). There also may be tech solutions, like Google Assistant making calls for people, that might be able to be pre programmed to make the call for him.

Nice that you're putting effort in, that alone is a huge help. I'm no expert in autism or even general cognitive disabilities, but your suggestions are pretty good, and not weird at all.